(a) Field of the Invention
The present invention relates to a system for controlling the flow rate of a refrigerant for use in air conditioners and refrigerating apparatus such as freezers, refrigerators and freezing or refrigerating showcases.
(b) Description of the Prior Art
Published Examined Japanese Patent Application SHO No. 58-47628 and the publication "Reito (Refrigeration)," Vol. 56, No. 641 (March 1981), pp. 60-64 disclose systems which are adapted to control the flow rate of a refrigerant in refrigeration cycles and which include a thermoelectric expansion valve. With such control systems, an electric signal is produced for controlling the degree of opening of the valve in accordance with the difference between an electric signal from a first temperature sensor provided at the inlet or an intermediate portion of an evaporator for refrigeration cycles and an electric signal from a second temperature sensor provided at the outlet of the evaporator to keep the difference constant and thereby maintain the degree of superheat (the temperature at the outlet of the evaporator minus the temperature at the inlet of the evaporator) approximately at a constant value for the control of the refrigerant flow rate. In actuality, however, the range of control thus afforded is narrow because the rectilinear portion of the flow rate characteristics is used for the control.
Further the thermostatic expansion valve generally used in refrigerating or air conditioning apparatus is such that the gas enclosed in its heat sensitive tube is ingeniously prepared so as to maintain the evaporator at a constant degree of superheat, but the valve is still narrower than the thermoelectric valve in the range of controllable capacities.
With the prior-art technique described above, the control capacity of the thermoelectric or thermostatic expansion valve differs when different kinds of refrigerants (for example, R-12, R-22 and R-502) are used and when the size of the space to be cooled (e.g. the interior capacity of refrigerator) changes. This gives rise to the necessity of selecting a valve which is suited to the object of control, entailing the drawback of requiring a very cumbersome procedure for the selection of the valve as well as for the adjustment of opening of the valve.
The present invention has been accomplished to overcome the foregoing drawbacks of the prior art.